1. Field of the Invention
The present invention relates to a semi-conductive roll which is suitably used as a charging roll, for example, in an image forming apparatus such as an electrophotographic copying machine, printer or the like.
2. Discussion of the Related Art
A semi-conductive roll such as a charging roll or a developing roll is installed in an image forming apparatus such as an electrophotographic copying machine, printer or the like, such that the semi-conductive roll is held in rolling contact with a photosensitive drum. For instance, the charging roll is used in a roll charging method wherein a photosensitive drum on which an electrostatic latent image is formed is charged by the charging roll. Described more specifically, in the roll charging method, the charging roll and the photosensitive drum are rotated such that the charging roll to which a voltage is applied is held in pressing contact with an outer circumferential surface of the photosensitive drum, to thereby charge the outer circumferential surface of the photosensitive drum. The developing roll carries a toner on its outer circumferential surface. The photosensitive drum and the developing roll are rotated such that the developing roll is held in pressing contact with the outer circumferential surface of the photosensitive drum on which the latent image is formed, so that the toner is transferred from the developing roll onto the photosensitive drum, whereby the latent image is developed into a visible image.
Since the charging roll and the developing rolls as described above are held in rotating contact with the outer circumferential surface of the photosensitive drum, such rolls are required to exhibit low hardness or high flexibility, and a moderate degree of electrical conductivity for charging the photosensitive drum.
In recent years, there is an increasing demand for higher image reproducing capability and excellent energy-saving characteristic of the image forming apparatus, as well as higher process speed and excellent durability of the image forming apparatus. In an attempt to improve the energy-saving characteristic for reducing the electric power to be consumed by the image forming apparatus, the melting point of the toner is lowered for the purpose of fixing the toner on a recording medium at a lower temperature. Further, for improving the performance of the image forming apparatus to provide sufficiently high image quality, the size of the toner particles is made smaller.
However, when the melting point of the toner is lowered or the size of the toner particles is made smaller, the toner undesirably tends to adhere to the semi-conductive roll such as the charging roll and the developing roll as described above which are inevitably heated in the image forming apparatus during the operation of the apparatus. The toner which adheres or clings to the semi-conductive roll causes deterioration of the image reproducing capability of the image forming apparatus. Described more specifically, with an increase in the number of copying or printing operations, in other words, with an increase in the number of operations to develop the electrostatic latent images into visible toner images on the outer circumferential surface of the photosensitive drum, the electric resistance of the semi-conductive roll such as the charging roll tends to be raised as a whole due to the adhesion of the toner to the semi-conductive roll. Further, uneven adhesion of the toner to local portions of the roll causes a variation in the resistance of the roll at the local portions. As a result, the image to be reproduced by the image forming apparatus is undesirably deteriorated.
As one measure for preventing the deterioration of the copying or printing quality, it is effective to prevent the toner from adhering to the surface of the semi-conductive roll such as the charging roll. However, a conventional roll whose outermost layer is formed of a hydrophilic resin such as N-methoxymethylated nylon suffers from variation of the electric resistance under the operation at high temperature and high humidity. In addition, the conventional roll is not capable of effectively preventing the adhesion of the toner to its surface. In place of such a nylon resin, a fluoro resin is used for forming the outermost layer of the roll since the fluoro resin permits relatively easy removal of the outermost layer from a mold used for forming the roll. However, the adhesion of the toner to the roll surface is not prevented to a satisfactory extent even in the roll whose outermost layer is formed of the fluoro resin as described above. Namely, with the increase in the number of copying or printing operations, the toner is likely to adhere to the roll surface. Thus, the conventional semi-conductive rolls do not exhibit sufficiently high durability.
It is therefore an object-of the present invention to provide an electrically semi-conductive roll which does not suffer from a variation of the electric resistance due to a change of the operating environment and adhesion of the toner to its surface, so as to avoid deterioration of the image to be reproduced and assure improved durability of an apparatus which includes the semi-conductive roll.
The above object may be attained according to a principle of the present invention which provides an electrically semi-conductive roll including a center shaft and a plurality of layers formed radially outwardly of the center shaft, wherein an outermost layer of the plurality of layers which is held in rolling contact with an outer circumferential surface of a photosensitive drum is formed by using a resin composition which contains as a base resin material a fluorine-modified acrylate resin.
In a first preferred form of the present invention, the resin composition further contains as the base resin material at least one of a fluorinated olefin resin and a fluorine-unmodified acrylate resin.
In a second preferred form of the present invention, the fluorine-unmodified acrylate resin has a plurality of hydroxyl groups, and the fluorine-unmodified acrylate resin is crosslinked by a crosslinking agent which reacts with the plurality of hydroxyl groups of the fluorine-unmodified acrylate resin.
In one preferred arrangement of the above second form of the present invention, the crosslinking agent is a polyisocyanate compound.
In a third preferred form of the present invention, the plurality of layers include an electrically conductive and soft base layer which is located radially outwardly of the center shaft and formed of an elastic body or a foamed body, a resistance-adjusting layer which is located radially outwardly of the electrically conductive and soft base layer, and a protective layer which is located radially outwardly of the resistance-adjusting layer and which functions as the outermost layer.
In one preferred arrangement of the above third preferred form of the invention, the semi-conductive roll further includes a softener-preventive layer which is located between the electrically conductive and soft base layer and the resistance-adjusting layer.
In a fourth preferred form of the present invention, the outermost layer has a volume resistivity of 106-1015 xcexa9xc2x7cm.
In the semi-conductive roll constructed according to the present invention, owing to the hydrophilic property of the fluorine-modified component of the fluorine-modified acrylate resin included as the base resin material in the resin composition for providing the outermost layer of the roll, the variation of the electric resistance of the roll which may be caused by the change of the operating environment is effectively reduced. In addition, the fluorine-modified component is effective to prevent various stains from adhering to the surface of the outermost layer, whereby the present roll does not suffer from adhesion of the toner to its surface. The fluorinated olefin resin which is used in combination with the fluorine-modified acrylate resin as described above effectively prevents various stains deposited on the roll surface from permeating therethrough into the inside of the roll, so that the stains deposited on the roll surface can be easily wiped off. Accordingly, even if the toner adheres to the surface of the outermost layer, it is easily removed therefrom owing to the inclusion of the fluorinated olefin resin as the base resin material in the resin composition for forming the outermost layer, so that the roll surface can be kept clean. The fluorine-unmodified acrylate resin which is used in place of, or in combination with the above-described fluorinated olefin resin is effective to improve adhesiveness or adhesion of the outermost layer to the underlying layer of the roll structure, leading to a significantly improved durability of the semi-conductive roll.
According to the above-described second preferred form of the present invention, the fluorine-unmodified acrylate resin which effectively improves the adhesiveness of the outermost layer has a plurality of hydroxyl groups. The plurality of hydroxyl groups of the fluorine-unmodified acrylate resin reacts with a predetermined crosslinking agent so as to introduce effective cross-linked structure, resulting in improved adhesion between the outermost layer and the underlying layer of the roll structure on which the outermost layer is formed. This arrangement effectively prevents peeling or separation of the outermost layer from the roll structure. Since the outermost layer of the semi-conductive roll is repeatedly deformed during a long period of use of the roll, the outermost layer tends to locally peel off away from the roll structure, which results in separation or cracking of the outermost layer. However, the outermost layer of the semi-conductive roll according to the present invention is formed by the resin composition which contains as the base resin material the fluorine-unmodified acrylate resin in which the cross-linked structure is introduced, so that the peeling or cracking of the outermost layer is advantageously avoided. As a result, the semi-conductive roll of the present invention does not cause the deterioration of the image reproducing capability which may arise from the peeling or cracking of the outermost layer. As the crosslinking agent, the polyisocyanate compound is preferably used for effectively introducing the cross-linked structure in the polymer of the fluorine-unmodified acrylate resin.
According to the above-described third preferred form of the present invention, the plurality of layers include an electrically conductive and soft base layer which is located radially outwardly of the center shaft and formed of an elastic body or a foamed body, a resistance-adjusting layer which is located radially outwardly of the electrically conductive and soft base layer, and a protective layer which is located radially outwardly of the resistance-adjusting layer and which functions as the outermost layer. It is preferable to provide the softener-preventive layer between the electrically conductive and soft base layer and the resistance-adjusting layer for the purpose of preventing a softener such as oil from bleeding from the electrically conductive and soft base layer.
According to the above-described fourth preferred form of the present invention, the outermost layer has a volume resistivity of 106-1015 xcexa9xc2x7cm, so that the semi-conductive roll effectively functions as a charging roll.